Here you would use the law of cosines.
a^2 = b^2 + c^2 - 2(b)(c)cosx
In this case we need to find the angle of f, so for a we use the length f is. It’s set up like this:
30^2 = 20^2 + 25^2 -2(20)(25)cosx
Next multiple and simply everything
900 = 1025 -1000cosx
Now subtract 1025
-125 = -1000cosx
Divide by -1000 to get cosx by itself
.125 = cosx
Now take the inverse of cos
Cos^-1 (.125)
And you get 82.8 or 83
So the answer should be A
Hope that helps. Lmk if there are questions.